According to the principles of the instant invention, there are disclosed herein improved photovoltaic devices which exhibit long term stability in the operating efficiency thereof and a method for the fabrication of said improved photovoltaic devices. That is to say, the photovoltaic devices of the instant invention are specifically designed so as to retain their initial operating efficiency (i.e., retain the percentage of incident photon energy converted to electrical energy) throughout their operating life, vis-a-vis, photovoltaic devices which degrade under the influence of incident photon energy due to "Staebler-Wronski effects", as will be fully explained hereinafter.
Recently, considerable efforts have been made to develop systems for depositing amorphous semiconductor materials, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n and n-i-p type photovoltaic devices which are, in operation, substantially equivalent to their crystalline counterparts. It is to be noted that the term "amorphous", as used herein, includes all materials or alloys which have no long range order, although they may have short or intermediate range order or even contain, at times, crystalline inclusions.
It is now possible to prepare amorphous silicon alloys by glow discharge or vacuum deposition techniques, said alloys possessing (1) acceptable concentrations of localized defect states in the energy gaps thereof, and (2) high quality electrical and optical properties. Such deposition techniques are fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors, issued in the names of Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; U.S. Pat. No. 4,217,374, issued in the names of Stanford R. Ovshinsky and Masatsugu Izu on Aug. 12, 1980, also entitled Amorphous Semiconductors Equivalent To Crystalline Semiconductors; and U.S. Pat. No. 4,517,223, issued in the names of Stanford R. Ovshinsky, David D. Allred, Lee Walter, and Stephen J. Hudgens on May 14, 1985 and entitled Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy. As disclosed in these patents, which are assigned to the assignee of the instant invention and the disclosures of which are incorporated by reference, fluorine introduced into the amorphous silicon semiconductor layers operates to substantially reduce the density of the localized defect states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was described at least as early as 1955 by E. D. Jackson in U.S. Pat. No. 2,949,498, issued on Aug. 16, 1960. The multiple cell structures therein disclosed utilized p-n junction crystalline semiconductor devices. Essentially, the concept employed different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc). The tandem cell device (by definition) has two or more cells with the light directed serially through each cell. In the first cell a large band gap material absorbs only the short wavelength light, while in subsequent cells smaller band gap materials absorb the longer wavelengths of light which pass through the first cell. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell, while the short circuit current thereof remains substantially constant. However, it is virtually impossible to match crystalline lattice constants as is required in the multiple cell structures of the prior art. Therefore, tandem cell structures cannot be practically fabricated from crystalline materials in a manner which would have commercial production potential. As the assignee of the instant invention has shown, however, such tandem cell structures are not only possible, but can be economically fabricated in large areas by employing amorphous materials.
The multiple cells preferably include a back reflector for increasing the percentage of incident light reflected from the substrate back through the semiconductor layers of the cells. It should be obvious that the use of a back reflector, by increasing the use of light entering the cell, increases the operational efficiency of the multiple cells. Accordingly, it is important that any photoresponsive layer of semiconductor material deposited atop the light incident surface of the substrate be transparent so as to pass a high percentage of incident light from the reflective surface of the back reflector through the photogenerative layers of semiconductor material of the cell(s).
Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Such continuous processing systems are disclosed in the following U.S. patents and pending application: U.S. Pat. No. 4,400,409, for A Method Of Making P-Doped Silicon Films And Devices Made Therefrom; U.S. Pat. No. 4,410,588, for Continuous Amorphous Solar Cell Production System; U.S. Pat. No. 4,438,723, for Multiple Chamber Deposition And Isolation System And Method; application Ser. No. 244,386, filed Mar. 16, 1981, for Continuous Systems For Depositing Amorphous Semiconductor Material; U.S. Pat. No. 4,492,181, for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells; and U.S. Pat. No. 4,485,125, for Method and Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these patents and application, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific semiconductor material. In making a photovoltaic device of p-i-n type configurations, the first chamber is dedicated for depositing a p-type semiconductor alloy, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor alloy, and the third chamber is dedicated for depositing an n-type semiconductor alloy. Since each deposited semiconductor alloy, and especially the intrinsic semiconductor alloy, must be of high purity, every possible precaution is taken to insure that the sanctity of the vacuum envelope formed by the various chambers of the deposition apparatus remains uncontaminated by impurities, regardless of origin.
The layers of semiconductor alloy material thus deposited in the vacuum envelope of the deposition apparatus may be utilized to form photoresponsive devices, such as, but not limited to, photovoltaic cells which include one or more p-i-n cells or one or more n-i-p cells, Schottky barriers, photodiodes, phototransistors, or the like. Additionally, by making multiple passes through the succession of deposition chambers, or by providing an additional array of deposition chambers, multiple stacked cells of various configurations may be obtained.
As should be apparent from the foregoing, thin film amorphous semiconductor materials offer several distinct advantage over crystalline materials, insofar as they can be easily and economically fabricated into large area photoresponsive devices by newly developed mass production processes. However, heretofore produced amorphous silicon based semiconductor materials from which photovoltaic devices were fabricated, proved prone to degrade as a result of prolonged exposure to light, especially during the operation thereof. This process, termed "photodegradation", or "Staebler-Wronski degradation" (mentioned hereinabove), although not fully understood, is believed to be due to the fact that long-term exposure of the amorphous semiconductor material to a photon flux tends to break weak bonds between the constituent atoms of the semiconductor material, thereby resulting in the formation of defect states in the band gap, such as dangling bonds, which are detrimental to the photovoltaic efficiency of a photoresponsive device from which degraded semiconductor material said photoresponsive device is fabricated. It has heretofore been observed that photogenerated defects may be annealed out of a sample of degraded semiconductor material by exposing said sample to elevated temperatures for a prolonged period of time; for example, a temperature of approximately 150.degree. degrees for several hours. In such a manner, samples of semiconductor material, thus degraded by operational exposure to light and subsequently annealed, may be restored to approximately the same level of photovoltaic performance which they exhibited prior to said operational degradation.
While somewhat parodoxical, it appears true that the higher the initial (pre-operational) quality of the semiconductor material from which a photovoltaic device is fabricated, (1) the greater the effect of photodegradation thereupon, and (2) the greater the operation-dependent loss in efficiency exhibited by said photovoltaic device incorporating such higher quality semiconductor materials. One possible explanation for this apparent parodox is that lower quality photovoltaic semiconductor material initially includes a relatively high number of defect states in the energy gap thereof and consequently, the formation of additional defect states in the energy gap thereof via the aforedescribed photodegradation process is not as significant as the formation of additional defect states is for a higher quality semiconductor material which is initially characterized by a relatively low number of defect states. Because of the fact that the assignee of the instant invention is now able to commercially manufacture high quality photovoltaic semiconductor materials exhibiting said low initial density of defect states in the energy gaps thereof, in a high volume, continuous production process; the problem of photodegradation of photovoltaic devices fabricated therefrom has become increasingly significant. As should be apparent, the practical ramification of significant operational losses in efficiency is to prevent the consuming public from purchasing photovoltaic energy generating systems (significant operational losses, for purposes of the instant application, will refer to losses upwards of 20% efficiency over the operating lifetime thereof).
Heretofore, the deleterious effects of photodegradation were dealt with by either (1) annealing the semiconductor material at relatively high temperatures for prolonged periods of time in order to remove the defect states in the energy gap thereof and restore the electrical generating capacity thereof to its original value, or (2) ignoring the defect states and allowing the semiconductor material to operate at less than full efficiency. Neither of the aforementioned options provides a commercially acceptable result which would stimulate consumer confidence and promote consumer purchasing of photovoltaic devices fabricated from that semiconductor material.
As to the first option, several methods of annealing have been employed. In one method the annealing procedure is instituted on a cyclic basis wherein the semiconductor material is periodically, typically at an interval of several months to a couple of years, heated to an elevated temperature for a period of time sufficient to substantially reduce the defect states present in the energy gap thereof and thereby substantially restore the initial photoconversion efficiency of the photovoltaic device which incorporates that degraded semiconductor material. The heating may be carried out in situ by incorporating a heat source in the photovoltaic installation itself, or the degraded semiconductor material may be removed from its installation and heated in an oven. Heating temperatures for such periodic annealing processes are typically 150.degree. C. or greater and the semiconductor material is maintained at that elevated temperature for at least a period of two hours.
In an alternative annealing process, the semiconductor material is continuously annealed by incorporating said material into a solar collector panel, (said collector panel referred to hereinafter as a "hot module") which panel is adapted to collect and retain the solar thermal energy incident thereupon. In such a solar heating arrangement, the semiconductor material is maintained, during normal operation, at an elevated annealing temperature (typically in excess of 100.degree. C.) and the defect states in the energy gap of the semiconductor material are annealed substantially simultaneously with the formation thereof. Depending upon the temperature maintained by the hot module during the operation of the photovoltaic device, the photodegradation of the semiconductor material can thereby be prevented or at least substantially reduced. Such methods and techniques of continuous high temperature annealing are disclosed in U.S. patent application Ser. No. 636,172 fo Vincent D. Cannella entitled, "Photovoltaic Panel Having Enhanced Conversion Efficiency Stability", filed July 31, 1984 and assigned to the assignee of the instant invention, the disclosure of which application is incorporated herein by reference.
While the use of the prior art high temperature annealing processes herein described do result in the substantial removal of photoinduced defects, the elevated temperatures required by said prior art processes impose limitations upon their utilization, and it is therefore desirable to lower the annealing temperature required to effect a substantial reduction in the number of photoinduced defect states in the energy gap of the semiconductor material. More particularly, in photovoltaic devices high annealing temperatures can result in damage to the busbars, grid patterns, and electrical circuitry associated with the photovoltaic device or the modules fabricated from a number of photovoltaic devices. Further, structures upon which the photovoltaic device or photovoltaic modules are deployed, such as roof tops, are readily susceptible to high temperature damage. Also, where continuous annealing processes, as described hereinabove, are employed, the design of the hot module may be greatly simplified if the annealing temperatures can be reduced.
As mentioned supra, in the second alternative of dealing with photoinduced defects, the amorphous photovoltaic devices which incorporate the semiconductor material may simply be allowed to photodegrade. The rate of photodegradation for a particular photovoltaic device configuration may be readily ascertained, and the power requirement for a given installation may therefore be readily specified to account for the degree of photodegradation expected during the operational life forecasted for the photovoltaic devices. For example, a particular photovoltaic device may be predicted to degrade to 80% of its initial electrical performance within a period of 10 years of operation; therefore, an excess capacity in electrical performance of the photovoltaic device of 20% may be incorporated in the initial installation to account for this subsequent loss or degradation. While such an approach is relatively simple and may be acceptable for a variety of photovoltaic installations, it obviously represents an inadequate solution to the problem (since it merely addresses the result and not the problem), and represents an intolerable solution for many other uses. In photovoltaic installations in which space for solar collection and electrical generation is at a premium, it is necessary to have the photovoltaic devices operate at peak efficiency at all times. In other photovoltaic installations reliability and consistency of electrical power generated by and delivered from the devices is required. In the latter type of installations, the photovoltaic devices must be fabricated from semiconductor material which is relatively consistent in its conversion efficiency throughout the expected operational lifetime thereof, i.e., minimum degradation over a 20 year period of time.
As previously stated, the mechanism of photodegradation of amorphous photovoltaic semiconductor materials is not fully understood; however, it is believed that said photodegradation involves the continuous production of a wide distribution of defect states in the band gap of the semiconductor material during operation of the photovoltaic device incorporating that semiconductor material. The term "defects", or "defect states", as generally used by routineers in the field of amorphous semiconductor materials, and as used herein, is a broad term generally including all deviant atomic configurations such as: broken bonds, dangling bonds, bent bonds, strained bonds, vacancies, microvoids, etc.
In a photovoltaic device, a charge carrier pair (i.e. an electron and a hole) is generated in response to the absorption of photons from incident radiation in the photoactive region of the semiconductor material thereof. Under the influence of an internal electrical field established by the doped layers of semiconductor material of the photovoltaic device, such as a solar cell, the charge carriers are drawn toward opposite electrodes of the cell causing the positively charged holes to collect at the negative electrode and the negatively charged electrons to collect at the positive electrode thereof. Under ideal operating conditions, every photogenerated charge carrier would be conducted to its respective collection electrode. However, operating conditions are not ideal and losses in the collection of photogenerated charge carriers occur to some degree in all photovoltaic devices. Note that the primary loss in charge carrier collection is due to charge carrier recombination, wherein an electron and a hole drifting through the semiconductor material toward an electrode of the photovoltaic device, find one another and reunite. Obviously, charge carriers that reunite or recombine are not available for electrode collection and the resultant production of electrical current. The aforementioned defects or defect states that occur in the photoactive region of the semiconductor material of the photovoltaic device provide recombination centers which facilitate the reunion and recombination of electrons and holes. Therefore, the more defects or defect states that are present in the semiconductor material of a device, the higher the rate of charge carrier recombination therein. Accordingly, charge carrier collection efficiency decreases as the rate of charge carrier recombination increases within the photoactive region of a given semiconductor material. Therefore, an increase in the number of defect states is at least partially responsible for an increase in the rate of charge carrier recombination and a concomitant decrease in the conversion efficiency of photovoltaic devices.
A further and different approach to the problem of photodegradation of thin film amorphous photovoltaic devices is disclosed in U.S. patent application Ser. No. 623,860 filed June 25, 1984 and entitled Stable Photovoltaic Devices and Method of Producing Same, which application is assigned to the Assignee of the instant invention and the disclosure of which is incorporated herein by reference. As disclosed therein, the semiconductor material of a photovoltaic device may be provided with a graded (i.e. varying) band gap in the photoactive region thereof, such that the widest band gap portion of that region is most proximate the light incident surface of the device and the narrowest band gap region thereof is distal of the light incident surface. In this manner, light will be absorbed more uniformly throughout the photoactive region of the semiconductor material resulting in a more uniform distribution of photogenerated carrier pairs throughout that photoactive region. A photovoltaic device, thus configured, is more tolerant of photoinduced defects, that is to say, the electrical output of the device will not be as severely degraded by the addition of photoinduced defects as that of a non-band gap graded photovoltaic device. It should be noted that while the band gap graded photovoltaic devices of the aforementioned application do not exhibit a lower number of light induced defects, the effect of the photoinduced defects on the operational efficiency of the devices is reduced by preventing a concentration of charge carriers adjacent the light incident surface thereof.
In contradistinction thereto, the instant invention relates to photovoltaic devices in which high operating efficiency is assured throughout the operational lifetime thereof by either (1) continuously or (2) periodically annealing out photoinduced defects at a relatively low and easily obtainable temperature. Therefore, the instant invention and application Ser. No. 623,860 are complementary. That is to say, they may be used in conjunction with one another to provide photovoltaic devices which not only exhibit low rates of photoinduced defect formation, but which also are characterized by the ability to continuously remove the effects of photoinduced defects which are formed through the innocuous vehicle of low temperature annealing.
As alluded to hereinabove, the instant invention relates to photovoltaic devices from which light induced defects may be annealed at relatively low temperatures. Such a low temperature annealing feature is provided in the devices by including small amounts of an annealing temperature reducing material, typically a dopant material, in the layer or layers of semiconductor material which form the photoactive (charge carrier generating) region of the device. In a typical p-i-n or n-i-p solar cell, formed of an amorphous silicon, or an amorphous silicon-germanium alloy, the inclusion of small amounts of a dopant material, such as boron, in the intrinsic semiconductor layer thereof has been found to lower the annealing temperature of said cells to 100.degree. C. or lower. This will be described in greater detail hereinbelow. In photovoltaic devices so configured, it becomes possible to substantially remove photoinduced defects by periodically heating the devices to temperatures of approximately 100.degree. C. for times of approximately fifteen minutes to one hour.
Alternatively, photovoltaic devices fabricated in accordance with the principles of the instant invention may be maintained in the continuous annealing mode of operation. It has been found that temperatures of as low as approximately 60.degree. C. are sufficient to anneal out photoinduced defects substantially as they form. Therefore, according to the principles of the instant invention, it is possible to fabricate a hot module type of photovoltaic device of relatively simple design, the semiconductor material of which will exhibit substantially no photoinduced degradation throughout the operational life thereof.
The literature has heretofore noted that the addition of boron to the layer of intrinsic semiconductor material of an amorphous silicon p-i-n type photovoltaic device resulted in an improved resistance to the formation of light induced defects therein; see for example, "Light Induced Instability of Amorphous Silicon Photovoltaic Cells" by S. Tsuda, et al, Solar Cells, 9 (1983) 25-36 and "Stability of P-I-N Hydrogenated Amorphous Silicon Solar Cells to Light Exposure" by Y. Uchida, et al Solar Cells, 9 (1983) 3-12. While it has been reported in the foregoing papers that the addition of relatively small amounts of boron to the layer of intrinsic semiconductor material of a p-i-n photovoltaic device improves the stability of said device, both of those papers specifically as well as the prior art, generally, have failed to recognize that dopant materials may be employed to lower the temperature at which light induced defects can be annealed out of the semiconductor material. It should further be noted that the doping levels employed by Tsuda and Uchida are generally lower than those utilized in the practice of the instant invention. Any improvement in the operational efficiency of photovoltaic devices produced in accordance with the methodology of Tsuda or Uchida is attributable to a change in the field profile of the photoactive region of the semiconductor material thereof which results from the addition of boron thereinto. Accordingly, the claimed improvement of Tsuda and Uchida's devices is attributable to the particular configuration of the devices (p-i-n, for example). Simply stated, the improvement is a device property, vis-a-vis, a material property (as is the improvement invented and described by Applicants herein).
More particularly, the instant invention effects an improvement in properties of the bulk photoactive region of the semiconductor material itself. In accordance therewith, there is disclosed by the instant invention, a semiconductor material which is specially designed so as to be provided with the capability of having light induced defect states in the energy gap thereof annealed therefrom at heretofore unattainable low temperatures. Furthermore, and in contradistinction to the Tsuda and Uchida work, the annealing properties of the semiconductor material are attained independently of the electrical field profile or any other device-related property. Additionally, the instant invention provides semiconductor material for the photoactive region of a photovoltaic device which is capable of annealing out a substantial percentage of light induced defects therefrom at low temperatures, while the device is operating. Therefore, such a device may be said to be essentially "self-healing" with regard to light induced defects, and is characterized by long term stability in the energy conversion efficiency thereof.
It may thus be seen that the instant invention has great utility insofar as it provides for the fabrication of a photovoltaic device from semiconductor material in which light induced defects are readily removed in a low temperature annealing process. The semiconductor material of such a photovoltaic device is readily restored from a lowered efficiency, photodegraded state to substantially, its initial operational state by a simple, energy efficient thermal annealing process which does not necessitate the use of relatively high temperatures capable of (1) damaging the underlying structures upon which the photovoltaic devices are deployed, or (2) harming personnel in the vicinity. Additionally, the low temperature annealable semiconductor material of the photovoltaic device can be readily adapted to a continuous annealing process by disposing the photovoltaic device in a module capable of operation at temperatures only slightly above ambient temperature.
Other objects and advantages of the instant invention will be more fully explained with reference to the Figures, the Detailed Description of the Invention and the claims which follow.